In photographic chemistry, sulfur sensitization with a sulfur compound, gold-sulfur sensitization with a gold-sulfur compound and gold sensitization with a gold compound are widely known as sensitizing methods for silver halide grains.
It is a well-known practice to chemically sensitize silver halide emulsion; electron microscopic observation of silver sulfide resulting from chemical sensitization has been reported, for example, by G. C. Furnell, P. B. Flint and D.C. Berch [Journal of Photographic Science, 25, 203 (1977)]. As reported in these reports, the size of fine grains of silver sulfide is very small of the order of several .ANG. to a few dozen .ANG., and they are abundantly present on silver halide grains.
The basic requirements of the performance of photographic silver halide grains are high sensitivity, low fogging and fine graininess.
A basic approach to the obtainment of high sensitivity emulsion comprising fine grains is to increase the photon efficiency in the light sensitizing process. Possible factors hampering the increase in photon efficiency include the presence of competitive electron traps originating from re-bonding, latent image dispersion, structural failure, lattice defects,etc. Sulfur sensitization and gold-sulfur sensitization are thought to act to provide the electron capturing center in the light sensitizing process. It is therefore important in sensitizing treatments to adjust the size, position and number of sensitivity specks serving as such light sensitization centers. Methods of controlling this position and number are reported or proposed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 9344/1986, 40938/1989, 62631/1989, 62632/1989, 74540/1989, 158425/1989, 34/1990 and 298935/1990.
However, all these methods aim at controlling the position of formation of the silver sulfide or gold-silver complex sulfide described above, and their size and number depend on the limited position (area), with no direct control of the size or number of silver sulfide, gold sulfide or complex nuclei thereof.
This is because the grains are very fine at the order of several .ANG. to a few dozen .ANG. as stated above, and because the size and number are significantly affected by the site and area of formation of silver sulfide on the silver halide crystal plane.
On the other hand, some methods have been proposed which are based on techniques different from those of ordinary chemical sensitization.
For example, Japanese Patent O.P.I. Publication No. 93447/1986 describes a sensitizing method in which not more than 10.sup.-3 mol/mol AgX fine grains of silver sulfide or gold sulfide are formed at specific points in silver halide crystals, but it gives no specific description about the size or number thereof.
Japanese Patent O.P.I. Publication No. 198443/1990 describes sensitization of silver halide grains by the addition of fine grains of silver sulfide zol thereto. However, this publication gives no description of the grain size of the silver sulfide zol obtained, describing nothing other than the luminescence spectrum of the silver sulfide zol. Nor is specified the grain size distribution. As recognized commonly, coloring with colloid grains varies widely depending on the size, chemical species and surface condition thereof; it is impossible to specify the size and distribution of colloid grains solely by their spectrum. Therefore, when this method is used, it remains unknown how many specks have been arranged on the silver halide crystal because the grain size is unknown.
As stated above, despite the fact that the size and number of fine grains of silver sulfide, gold sulfide, etc. on silver halide crystals are critical factors in the light sensitizing process, they remain out of control. This is because their size is too small; to date, no one has ever succeeded in controlling the size and number of these grains.
On the other hand, assemblies of 2 to a few hundred atoms, called microclusters, are known, whose substance phase is a transition phase not categorized under any of solid, liquid and gas, in which almost all atoms are located on the surface of the basket-like assembly. For this reason, some electrons are not involved in bonding but generally active in a non-localized state, making the assembly highly reactive at points of particular numbers of atoms (called the magic numbers of microclusters). In short, the size of cluster plays a key role in the activity thereof.
In the 1980's, Richard E. Smalley, Vlandimir E. Bondybey et al. succeeded in forming a cluster by the laser evaporation method.
Another microcluster, stable as a Synthol compound ion, is also known.
Although much remains unknown as to these microclusters and behavior thereof, they draw attention as providing an important hint for silver halide sensitization, sensitivity speck formation in the light sensitizing process, sensitivity speck behavior or growth to developing specks, etc.